Results 1 to 6 of 6

Thread: Grundfos alpha pump too slow on autoadapt, CP or PP settings

  1. #1
    Automated Home Sr Member
    Join Date
    May 2021
    Posts
    55

    Default Grundfos alpha pump too slow on autoadapt, CP or PP settings

    I remember seeing some chatter, it might have been on another forum, about the auto-adapt and CP/PP modes on these Alpha 2 pumps, in the end I and others ended up using one of the fixed speed modes (mine is set on II). The problem seems to be that the pump runs too slowly in the auto-adapt, CP or PP modes, the water overheats causing the boiler to cycle, and my rooms never get up to the set point. If I use fixed speed II then the water runs fast enough that the boiler stays lit, modulates down and the rooms quickly get up to their set points.

    I had a problem yesterday where the boiler wouldn't fire when the only HR92s open where my bypass rad in the bathroom (which doesn't have a TRV) and my rad in the office where I work. So I applied a bit of thinking and thought "hmm I wonder if it's the flow rate"... so I opened the lockshield a couple turns on my bypass rad and sure enough the boiler fired and I started getting heat into the office radiator.

    So this got me thinking a bit more about the pump problem... maybe I need to try the CP or PP setting again (looks like they recommend autoadapt or PP2) but this time open the lockshields and re-balance all the radiators - I wonder if this will get the flow rate up enough to allow the boiler to fire and stay lit. It will take a while but when only 1 or 2 rads are open I do hear quite a bit of water "whooshing" the open valves so I would prefer for the pump to back off a bit and it does bug me that I have an "intelligent" pump but I am using it in a non-intelligent way.

    Has anyone else done this?

  2. #2
    Automated Home Legend
    Join Date
    Sep 2014
    Location
    Scotland
    Posts
    2,337

    Default

    I had no luck with the variable speed modes on my Alpha 2 - same as you, runs too slowly and the boiler overheats, so I run it in fixed speed 1 or 2.

    I think the variable speed modes have a relatively narrow criteria for system flow resistance to work properly. If you have too much flow resistance the pump in variable speed mode will just slow right down until it is hardly turning.

  3. #3
    Automated Home Legend
    Join Date
    Jul 2014
    Posts
    1,630

    Default

    Same here. Setting it to highest fixed speed was my only solution.

  4. #4
    Automated Home Sr Member
    Join Date
    May 2021
    Posts
    55

    Default

    Quote Originally Posted by DBMandrake View Post
    I think the variable speed modes have a relatively narrow criteria for system flow resistance to work properly. If you have too much flow resistance the pump in variable speed mode will just slow right down until it is hardly turning.
    So would the solution be to reduce the resistance by opening all TRVs and all the lockshield valves? If it works and the boiler stays lit then it's a case of slowly rebalancing all the rads? - it does sound like a complete pain in the arse though.

    I guess the flow rates will change as the the TRVs close so could end up with the boiler cycling again if it drops too low, so you might get, say, half the rads balanced and find that it becomes impossible to keep the boiler lit for the last few... ?

  5. #5
    Automated Home Legend
    Join Date
    Jul 2014
    Posts
    1,630

    Default

    You are suggesting what I have tried and found impossible to achieve with Evohome. However I have another problem in that my boiler has landed up getting oversized due to changes in our heating configuration. That added another complexity which meant that my pump had to run fast to push the hot water away from the boiler.

  6. #6
    Automated Home Sr Member
    Join Date
    Dec 2017
    Posts
    72

    Default

    Boiler's have a minimum flow rate at varying power outputs. For Vaillant's (mine is a 438) this is particularly important because the boiler takes 30s or so before it modulates down (it starts at X% of full power, dependant on firmware).

    In order to maintain this minimum flow rate, you'll need to know the index circuit for your central heating (the one with the highest resistance) and ensure you have an auto bypass valve set correctly to allow the minimum flow rate. The pump curve will then need to be used to ensure the PP setting allows the same minimum flow to overcome the head of the system.

    All of it is a compromise, though, and very difficult to predict exactly how each system will behave as there's always some unknown variables. Eg. TRV shutting down will increase the resistance of the index circuit.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •